The Uninfected Achievement
Finish a Ranked Infestation match as the only remaining prey
One player is an Alien and once it kills a Marine, the player becomes an Alien, too. So use your weapons wisely and be cautious. The best tactic is looking for a remote place and hoping that the aliens go straight for the pack. If you manage to stay hidden there, until you are the last man standing your chances are getting quite high to survive. Many people quit after they've become the alien.